An armor piercing projectile comprises a projectile body, a penetrator and a windshield. Subcaliber penetrators are used in armor piercing projectiles to obtain increased penetration performance compared to a penetrator round of full diameter. The conventional design of the subcaliber penetrator is a cylinder having a length-to-diameter ratio in the range of 6 to 8 with various nose shape configurations, including ogival, conical, and double conical. The reason that penetration performance is improved by the subcaliber penetrator is that more energy per unit area can be applied at the target. Maximum performance is achieved in conventional subcaliber penetrator designs by using the highest density materials for the penetrator which maximizes the energy per unit area.
In spin stabilized ammunition, a limit is reached on the allowable length-to-diameter ratio for the penetrator because of the increased spin required to stabilize the longer penetrators. Because the length for a given diameter is limited, the energy per unit area of the penetrator is also limited for a given penetrator material. The present invention provides a special design of the subcaliber penetrator which removes the performance limitation discussed above and allows the design of penetrator rounds within stability limitations that deliver higher energy per unit area at the target than do conventionally designed subcaliber rounds for the same stability restraints.
The gyroscopic stability factor of a projectile is directly proportional to the quotient of the axial moment of inertia squared divided by the transverse moment of inertia: ##EQU1## WHERE S.sub.G = stability factor (must be greater than one for gyroscopic stability)
K = proportionality constant (can be considered constant only if comparing projectiles of identical exterior shape and total weight) PA1 I.sub.a = axial moment of inertia PA1 I.sub.t = transverse moment of inertia PA1 y = radial distance from longitudinal axis PA1 a /b & b are constants PA1 x = distance measured from the tip PA1 N = exponent defining the curved shape
The difference between the invention and conventionally designed subcaliber penetrators can be shown by analysis of the above equation. For the conventional subcaliber penetrator the length-to-diameter ratio and, therefore, the energy per unit area at target impact is limited by the moment of inertia considerations. The smaller diameter of the conventional subcaliber penetrator reduces I.sub.a .sup.2 and the increased penetrator length increases I.sub.t ; both changes are cumulative and reduce S.sub.G. The penetrator of the instant invention has a maximum length-to-diameter ratio of 5:1 and has a shape whose envelope from tip to base is substantially defined by an exponential curve of revolution about its longitudinal axis. Said exponential curve of revolution is described by the formula: EQU Y = a + b x .sup.N
The exponent range for the invention must be greater than one and thus the surface is concave. Conventional subcaliber penetrators have the conical or ogival shape which could also be defined by the exponential curve of revolution, however, the exponent would be one or less and, therefore, the surface would be conical or convex rather than concave. It has been found that conventional designs having length-to-diameter ratios greater than 5 tp 1 have undesirable stability characteristics. In the penetrator shape of the instant invention, the material toward the base that is at a larger diameter than the forward tip nose section acts to increase the axial moment, I.sub.a as compared to the condition where this material was added to the base at the same diameter (as in a conventional high L/D projectile). The position of this material also is closer to the center of gravity of the projectile. Thus the transverse moment of inertia (I.sub.t) is reduced. Both moment changes according to the above equation, increase the stability factor, S.sub.G, over the conventional high L/D projectile. The invention gives performance at the target of a high length-to-diameter ratio penetrator, however, the actual moment of inertia considerations do not degrade the stability of the projectile because of its special design.